The genetic material of all organisms is subject to a daily barrage of physical and chemical insults coming from the environment or produced by normal cellular metabolism. Such genotoxic agents change the chemical structure of the components of DNA and hence its coding properties and set the stage for subsequent events that lead to a variety of potentially deleterious biological consequences including cell death, impaired function, mutation, and in the case of humans, the development of cancer. This revised Program Project proposal is comprised of five research projects focused on the cellular responses to genotoxic stress from agents that are representative of major classes of environmentally relevant chemicals and radiation as well as agents, such as reactive oxygen species, (ROS) which may be generated spontaneously within cells. In this regard, an emphasis will be placed on responses to DNA damage caused by oxidizing, alkylating, and nitrosating agents as well as radiation. Humans and other organisms are exposed to such agents as a consequence of industrial, agricultural and lifestyle activities. Over the past several years, a critical mass of investigators with common interests in the area of DNA damage, resistance have assembled at Emory University. The five principal investigators of this Program Project represent a majority of this critical mass. These groups have a history of productive interactions and collaborations. It is anticipated that continued interactions as well as the development of new collaborations will further synergize the efforts of the groups, propelling the pace of discovery significantly and allow these investigators to reach new levels of comprehension of how DNA repair, damage tolerance, and direct damage prevention pathways are intertwined. Yeast and bacterial model systems will be employed to address several important aspects of strategies employed by organisms to resist the introduction and effects of DNA damage. The DNA damage resistance strategies that will be analyzed will include DNA repair, damage tolerance and direct prevention pathways. The broad theme of this revised Program Project is the characterization of the interacting and complementary networks of systems that operate at different levels during the cellular response to exposures to DNA damaging agents. In pursuing this Program, it is anticipated that the investigators' understanding of DNA damage processing in cells will be brought to a significantly higher level of resolution.